Dio-Xiphilinus, Herodian, and the Historia Augusta give three apparently contradictory accounts of the circumstances in which Commodus' all-powerful praetorian prefect, Sex. Tigidius Perennis, was overthrown in A.D. 185. My purpose here is not to try to decide between them, but primarily to correct what I think a patent misinterpretation, now current, of a crucial statement in that given by Xiphilinus in his epitome of Dio.